A spark plug including a pre-chamber for a combustion chamber of an engine is known. This type of spark plug includes a plug cap that is connected to a metal shell and that has a through hole. The plug cap is exposed in the combustion chamber so that the pre-chamber is provided in the combustion chamber. The spark plug ignites combustible air-fuel mixture that has flowed into the plug cap from the combustion chamber through the through hole. The combustible air-fuel mixture is combusted to generate an expansion pressure that causes a gas flow including flame to be injected into the combustion chamber through the through hole. The combustible air-fuel mixture in the combustion chamber is combusted by the injected flow of flame. Japanese Unexamined Patent Application Publication No. 2006-144648, hereinafter “patent document 1” (in particular, FIG. 26) discloses a spark plug including a plug cap having an inner surface on which ridges are formed in a region behind through holes so that the cross-sectional area of the pre-chamber gradually increases toward the back.
However, according to the technology disclosed in patent document 1, when the combustible air-fuel mixture flows into the plug cap from the combustion chamber through the through holes, the plug cap, which is exposed in the combustion chamber, is cooled by the combustible air-fuel mixture. In particular, the temperature of a front portion of the plug cap becomes lower than the temperature of a back portion of the plug cap. As a result, the temperature of the combustible air-fuel mixture in a front region in the plug cap is reduced. Accordingly, in the region around the front end of the plug cap, the flame propagation velocity is reduced in accordance with the reduction in the temperature of the combustible air-fuel mixture. When the velocity of flame propagation from the inside of the plug cap toward the through holes is reduced, the combustion rate in the combustion chamber is adversely affected.